1. Field of the Invention
The present invention relates to a primary radiator provided, for example, in a reflector type antenna for satellite broadcast reception. Particularly, the invention is concerned with a primary radiator suitable for a reflector having a reflective surface which is not circular.
2. Description of the Prior Art
In the case where a primary radiator is disposed at a focal position of a reflector in a satellite broadcast receiving reflector type antenna, it is necessary, for efficiently receiving a radio wave from a satellite, that the shape of a reflective surface of the reflector and a radiation pattern of the primary radiator be matched. Usually, for this reason, in the case where the reflective surface of the reflector is in a non-circular shape such as an elliptic or rectangular shape, there is used a primary radiator wherein an aperture of a horn portion as a radio wave inlet is elliptic in shape.
FIG. 9 is a perspective view showing a conventional primary radiator of this type and FIG. 10 is a side view of the primary radiator as seen in an aperture direction of a horn portion. This primary radiator is provided with a horn portion 1 having an elliptic aperture 1a, a waveguide 2 of a circular section contiguous to the horn portion 1, and a dielectric plate 3 and a probe 4 both disposed in the interior of the waveguide 2. The horn portion 1 and the waveguide 2 are integrally formed, for example, by aluminum die casting or zinc die casting. The dielectric plate 3 has predetermined dielectric constant and shape and functions as a phase compensating portion which offsets a propagative phase difference based on a difference between a minor axis and a major axis in the aperture 1a of the horn portion 1. The probe 4 picks up a polarized wave which has been phase-compensated by the dielectric plate 3 and it is spaced a distance corresponding to about one fourth of the wavelength in waveguide from an end face 2a of the waveguide 2.
The primary radiator thus constructed is disposed at a focal position of a reflector having a reflective surface of a non-circular shape in a satellite broadcast receiving reflector type antenna. But a linearly polarized wave transmitted from a satellite has a predetermined polarization angle due to a positional relation to the place where the antenna is installed. For example, in case of receiving al linearly polarized wave from an ASTRA satellite in the suburbs of London, England, the linearly polarized wave has a polarization angle of about 13xc2x0. In this connection, since a reflector having an elliptic or rectangular reflective surface is installed horizontally with respect to the surface of the earth so as not to spoil the appearance thereof, a linearly polarized wave reflected by the reflector becomes incident in an inclined state with respect to the minor axis and major axis of the aperture 1a in the horn portion 1. When the polarization plane (an incident field polarization plane 5) of the incident radio wave is thus inclined relative to the minor axis and major axis of the elliptic aperture 1a, as shown in FIG. 10, the radio wave which has passed through the horn portion 1 becomes an elliptically polarized wave having a phase difference induced by an incident field minor axis component 6 and an incident field major axis component 7, which elliptically polarized wave is introduced into the waveguide 2. Also in the interior of the waveguide 2 there is induced a phase difference by both a component parallel to the dielectric plate 3 and a component perpendicular thereto. However, since this phase difference induced under the influence of the dielectric plate 3 and the foregoing propagative phase difference based on the minor-major axis difference in the aperture 1a of the horn 1 are set at a mutually offset relation, the elliptically polarized wave which has entered the interior of the waveguide 2 becomes a linearly polarized wave when passing through the dielectric plate 3 and is propagated to the innermost part of the waveguide. Then, for example a vertically polarized wave contained in the linearly polarized wave is received by the probe 4 and the received signal is frequency-converted into an IF frequency signal in a converter circuit (not shown), which IF frequency signal is outputted.
In the conventional primary radiator constructed as above, since the horn portion having the elliptic aperture 1a is formed in one piece with the waveguide 2 by, for example, aluminum die casting or zinc die casting, the manufacturing cost, including the cost of the mold used, becomes high and the size of the primary radiator becomes large. Moreover, although the propagative phase difference induced in the horn portion 1 is offset by the dielectric plate 3 mounted in the interior of the waveguide 2, if the dielectric plate 3 is not accurately mounted with respect to the minor and major axes of the horn portion 1, the dielectric plate 3 does not fulfill its function as a phase compensator to a satisfactory extent and there occurs a marked deterioration of the cross polarization characteristic.
The present invention has been accomplished in view of such actual circumstances of the prior art and it is an object of the invention to provide a primary radiator which is less expensive and suitable for the reduction of size and which can positively prevent the deterioration of a cross polarization characteristic.
For achieving the above-mentioned object, the primary radiator of the present invention comprises a waveguide having a radio wave introducing aperture at one end thereof and a dielectric feeder held in an aperture end of the waveguide, the dielectric feeder being provided with a radiating portion having different radiation angles in two-axis directions orthogonal to each other, a phase compensating portion for compensating a propagative phase difference in two-axis directions induced in the radiating portion, and a converting portion for impedance-matching a radio wave between it and the waveguide.
With use of such a dielectric feeder, it is not only possible to shorten the overall length of the primary radiator, including the radiating portion, but also possible to simplify the shape of the waveguide and thereby reduce the manufacturing cost. Besides, since the radiating portion and the phase compensating portion are integrally provided in the dielectric feeder, the propagative phase difference induced in the radiating portion is sure to be offset in the phase compensating portion and it is possible to positively prevent the deterioration of a cross polarization characteristic.
In the above construction it is preferable that the radiating portion be formed in a wedge or horn shape. Particularly, if a plurality of annular grooves having a depth corresponding to a quarter wavelength of a radio wave are formed in an end face of the horn-shaped radiating portion, the radio waves reflected by both end face of the radiating portion and bottoms of the annular grooves are phase-cancelled and therefore can be converged efficiently to the radiating portion.
As the phase compensating portion in the above construction there may be adopted any of various forms. For example, there may be adopted a construction in which an outer peripheral surface of the dielectric feeder is cut out to form a pair of flat surfaces so that the flat surfaces are opposed to each other in parallel in the major axis direction of the radiating portion, thereby constituting a phase compensating portion.
Alternatively, there may be adopted a construction wherein a cavity is formed in the interior of the dielectric feeder so as to be in a long and slender shape in the major axis direction of the radiating portion, to constitute a phase compensating portion. In this connection, if the foregoing converting portion is constituted by a stepped hole comprising a plurality of axially contiguous recesses, the recesses each having a quarter wavelength of a radio wave, it is preferable that at least one of the recesses also function as a phase compensating portion.
Alternatively, there may be adopted a construction wherein a projecting portion is formed at an end face of the dielectric feeder on the side opposite to the radiating portion so as to be in a long and slender shape in the minor axis direction of the radiating portion, thereby constituting a phase compensating portion. In this connection, if the converting portion is constituted by a stepped projection comprising a plurality of axially contiguous projecting portions, the projecting portions each having a height corresponding to a quarter wavelength of a radio wave, it is preferable that at least one of the projecting portions also function as a phase compensating portion.